In drilling an oil or gas well, successive measurements are typically made of various borehole conditions and properties of earth formations that are being penetrated by a drill bit as it progressively creates a borehole. In the past, many of these measurements could not be obtained unless the drill string was temporarily removed from the borehole and one or more wireline logging operations were conducted in the open borehole. Since wireline logging operations can significantly prolong the time needed to complete a borehole, the usual practice is to minimize the number of open hole logs that are run during the course of drilling a given borehole.
Different proposals have been made for making one or more of these measurements without having to remove the drill string. For example, U.S. Pat. No. 3,112,442 discloses a self-contained instrument including a suitable power supply, a recorder, and various electrical and/or radioactivity sensors that is adapted to be moved through the longitudinal bore of a drill string and landed on a seat just above the drill bit. Once a series of measurements is taken, a wireline overshot is lowered through the drill string and coupled to a fishing neck on the upper end of the instrument housing for returning the instrument to the surface for analysis of the measurements.
A similar arrangement is shown in U.S. Pat. No. 3,209,323 which transmits recorded measurements to the surface by way of a logging cable carrying an overshot having a winding inductively coupled to a matched winding in a fishing neck on the instrument. Another proposal in U.S. Pat. No. 3,186,222, employs a self-contained measuring assembly having electrical and/or radioactivity sensors that are mounted on the lower end of the drill string just above the bit. With this arrangement, the output signals from the measuring assembly are converted into successive alternately-polarized electromagnetic pulses that are transmitted along the walls of the drill string to surface detectors by means of self-contained repeater stations tandemly coupled at spaced intervals in the drill string. Since the radioactivity logging devices disclosed in the above-cited patents are designed to measure only the natural gamma radiation from the earth formations, such logging devices do not require a source of radioactivity.
Man of the problems associated with various prior systems have been at least partially overcome with various measuring-while-drilling (MWD) or logging-while-drilling (LWD) tools. With the introduction of MWD tools that are now commercially available, it has become practical to transmit to the surface one or more real-time downhole measurements without interrupting the drilling of a borehole. As described, for example, in greater detail in U.S. Pat. No. 3,855,857, the MWD tool disclosed therein can measure downhole conditions such as weight-on-bit, torque acting on the bit, azimuthal direction and angle of inclination of the borehole, borehole pressure and temperature, mud resistivity, and various characteristics of the earth formations penetrated by the bit. The output signals of the various sensors are coupled to circuits that selectively control a downhole acoustic signaler in the tool for successively transmitting encoded data signals representative of these real-time measurements through the mud stream in the drill string to suitable detecting and recording apparatus at the surface.
Several MWD tools have been proposed for providing real-time measurements of different radioactivity characteristics of the earth formations being penetrated by the drill bit. Since measurement of natural gamma radiation requires only a gamma-ray detector and typical circuits to control the signaler, it has not been difficult to provide MWD tools with that instrumentation. Typical MWD tools with this capability are shown, for example, in U.S. Pat. No. 4,520,468 as well as in FIG. 4 of U.S. Pat. No. 3,255,353. On the other hand, as depicted in FIG. 1 of the last-cited patent, to measure other radioactivity characteristics of earth formations, an MWD tool must also have an appropriate source of radiation such as a radioactive chemical source. Since the measurement of formation density is impaired by borehole fluids, as seen, for example, in U.S. Pat. No. 4,596,926, it has been proposed to compensate for the effect of the fluids by arranging an array of radioactive sources and radiation detectors around the tool body.
Another prior attempt at providing nuclear formation evaluation while drilling is disclosed in U.S. Pat. Nos. 4,596,926; 4,698,501; and 4,705,944. In these patents, the disclosed apparatus design places the nuclear source in pockets located on the exterior of a drill collar. Three independent measures are provided for preventing the source from becoming dislodged during the drilling operation. While these measures appear to reduce the risk of losing a source in the well, it is believed that the process of inserting and removing the source into and from the pocket requires a considerable amount of time and effort, which might expose the operators performing the task to a significant amount of radiation. In addition, the amount of time required to remove the source from the tool upon return to the earth's surface can be significantly increased if the drilling process and downhole environment has damaged the exterior of the drill collar and/or the source pocket.
Any time an MWD tool is being used in a drilling operation, it is always possible for the tool to become inadvertently stuck in the borehole. If the tool or drill string becomes firmly stuck, it may be necessary to back-off and retrieve the upper portion of the drill string and then to use one or more "fishing" techniques to recover the remaining portion of the drill string including the MWD tool from the borehole before the drilling operation is resumed. Such fishing operations may, however, subject an MWD tool to severe impacts that might severely damage the tool's inner components. Thus, should an MWD tool carrying a chemical radioactive source become stuck to such an extent that the tool cannot be readily recovered, it is possible that the containment means around the source might rupture in the course of a fishing operation. Should this occur, the borehole fluids could become contaminated, thereby creating a hazard in handling the damaged tool when it is recovered at the surface, or if the borehole fluids are circulated back to the rig and associated surface equipment. In addition, if the source-containing MWD tool cannot be recovered, various expensive environmental protection procedures must be followed. Accordingly, such risks must always be considered when the conditions for a borehole are such that a source-carrying MWD tool might become stuck.
To overcome such problems, various proposals have been made to provide self-contained instruments that can be moved through the drill string and temporarily stationed in one of the drill collars just above the drill bit. For example, as described in U.S Pat. No. 4,041,780, a self-contained logging instrument is arranged to be pumped through the drill string to a landing seat temporarily installed on the lower end of the drill string. However, it is believed that a major disadvantage with this instrument is that the drill bit must be temporarily replaced with the sub carrying the seat when a series of measurements are to be made. Thus, since each series of measurements requires two complete round trips of the entire drill string, ordinarily it will be far more practical to use a wireline logging tool for obtaining these measurements while the drill string is out of the borehole.
U.S. Pat. No. 4,550,392 also describes a similar self-contained instrument that is moved into and out of the drill string by a cable. However, even though this instrument is installed and removed while the drill string and drill bit are in the borehole, because the instrument's sensors are located within a thick-walled drill string, certain formation radioactivity characteristics can not be effectively measured. Moreover, once the instrument has been removed from the drill string, the drilling operation must be continued without the benefit of further downhole measurements.
In commonly-assigned U.S. Pat. Nos. 4,814,609; 4,845,359; and 4,879,463; which are incorporated herein in their entirety by reference, various embodiments of measurement-while-drilling nuclear tools are disclosed that are adapted to be coupled to a drill string to make formation evaluations. One noteable feature of these tools is the provision of a retrievable source carrier that includes one or more radioactive sources and which is cooperatively arranged to be moved through the interior of a drill string to a selected station within the tool's body. The source carrier preferably includes a relatively stiff metal cable joining the radioactive sources. By providing such a source carrier, it is possible to insert or remove the sources into or from the tool while the tool is positioned below the drilling rig floor, which substantially avoids high radiation conditions at the rig floor. In addition, such a source carrier makes it possible to remove the radioactive sources from the tool if the tool were to become stuck within the borehole. A wireline conveyed overshot is used, for example to retrieve the source carrier.
The MWD tool and retrievable source carrier disclosed in these patents are extremely reliable and provide safety measures not found elsewhere. However, it has been found that with the present design of the source passageway and source receptacle area within the tool, solid particles commonly found in the drilling mud can enter the source passageway and receptacle area and become packed around the source carrier. In some cases, these particles can make it somewhat difficult to remove the source carrier from the tool. In addition, it has been found that in some instances the lower source receptacle can become partially filled with particulates found in the drilling mud which prevent the lower source from being properly positioned within the receptacle, which in turn raises the possibility of producing inaccurate formation measurements. The possibility of improper source positioning is further exaggerated by the fact that the cable joining the sources is subject to compressive deformation, i.e. "birdcaging", if solids partially block or settle within the lower receptacle area and prevent the lower source from fully reaching its proper position within the lower source receptacle.
In light of the above, a principle object of the present invention is to provide an LWD apparatus having one or more radiation detectors cooperatively arranged within a tubular body to measure one or more radioactivity characteristics of adjacent earth formations with a retrievable source carrier assembly that can be more easily inserted into or removed from the tubular body in such a manner that personnel on the drilling rig are exposed to as little radiation emitted from the sources as is reasonably possible.
A further object of the present invention is to provide a nuclear LWD apparatus with a retrievable source carrier assembly that can be more easily removed and recovered from the tool during a drilling operation in the event that the drillstring and LWD apparatus become stuck within the well.
Another object of the present invention is to provide a nuclear LWD tool with an internal source passageway and source receptacle area that substantially resist the accumulation of mud-carried particulate buildup in operation.
Another principle object of the present invention is to provide a source carrier assembly for use in a nuclear LWD tool wherein the sources are joined by a solid yet flexible member having a substantially smooth outer surface that substantially resists the accumulation of particulate buildup thereon.
Yet another object of the present invention is to provide a source carrier assembly for use in a nuclear LWD tool wherein the sources are joined by a solid yet flexible member having a high compressive strength that will not deform when subjected to normal compressive loads.